Doc.: IEEE 802.22-09/0068r2



IEEE P802.22

Wireless RANs

|Sensing performance with the 802.22.1 wireless microphone beacon |

|Date: 2009-07-15 |

|Author(s): |

|Name |Company |Address |Phone |email |

|Gerald Chouinard |CRC |3701 Carling Ave. Ottawa, Ontario Canada |613-998-2500 |gerald.chouinard@crc.ca |

| | |K2H 8S2 | | |

Sensing performance with the 802.22.1 wireless microphone beacon

1- Wireless microphone operation and sensing

Wireless microphones typically transmit with a 10 mW conducted power in the UHF TV band but practical considerations limit the antenna gain to about –10 dBi, resulting in 0 dBm EIRP. The range of typical wireless microphones is 100 m from the receiver. In practice, these wireless microphone applications assume an attenuation, beyond the normal free-space propagation, of up to 27 dB due to body absorption and multipath, resulting in a minimum signal level present at the wireless microphone receiver of –95 dBm at 100 m. Protection against interference is needed down to this level with an assumed desired-to-undesired ratio (D/U) of 20 dB. [1]

2- Wireless microphone sensing

The FCC R&O 08-260 specified that wireless microphone signals should be detected at a level as low as –114 dBm, assuming a 0 dBi sensing antenna gain and no cable loss. This sensing threshold corresponds to a successful detection of a wireless microphone at 1 dB SNR for a 6 dB sensing RF front-end noise figure in a 200 kHz bandwidth or –14 dB SNR in a 6 MHz bandwidth. [1]

Tests carried out by the FCC OET in 2008 [2] reported problems with local spurious signals erratically triggering the wireless microphone sensing. This is made evident by the fact that this –114 dBm sensing threshold is 19 dB lower than the signal level produced in 200 kHz by an electronic device meeting the out-of-band emission level prescribed in Part 15.209(a) at a separation distance of 10 m. In fact, the sensing device would need to be located at a minimum of 67 m from any electronic equipment in line-of-sight to avoid a desensitization of more than 1 dB for its 200 kHz wide microphone detector, something that cannot be ensured in practice. [1]

If the wireless microphone detector operates over a 6 MHz bandwidth, the level of spurious and out-of-band emission allowed in Part 15.209a is 33.5 dB above the specified –114 dBm sensing threshold for any radiating device located at a distance of 10 m. In this case, the 1 dB desensitization level of the detector would result from the presence of an electronic device at a distance of 364 m in line-of-sight condition. The –114 dBm threshold is therefore clearly in the range of man-made noise.

It was also found that wireless microphones tend to increase the noise level in adjacent and alternate TV channels with the potential of masking the presence of other wireless microphones operating in these channels, or producing a denial of service for TVBDs trying to operate in all these TV channels if presence of RF noise in a TV channel is sufficient to assume that a broadcast incumbent is present.

3- The 802.22.1 wireless microphone beacon

The IEEE 802.22 Working Group developed a specific standard for a digitally modulated beacon signal to be used to indicate the presence of nearby wireless microphone operation to unlicensed devices operating in the TV white space. This development was done in Task Group 1 of the 802.22 Working Group and this is the reason why the standard was numbered 802.22.1. This standard has now gone through its first round of Sponsor ballot and should reach a formal standard status in the coming months. [3]

This wireless microphone beacon standard was developed especially to operate as a Part 74 device at 250 mW and over a bandwidth of 77 kHz in a specific microphone sub-channel in a TV channel to signal the presence of wireless microphone operation in this channel in the area. The 802.22 Draft Standard has also been enhanced to allow scheduling of different lengths of quiet period to allow sensing and acquisition of various information from the 802.22.1 beacon signal as well as the necessary MAC messages for the base station (BS) to request sensing and the customer premise equipment (CPE) to report the results to the base station. [4]

Special care was taken in selecting the beacon modulation and bandwidth so that Part 74 wireless microphones can be protected from unlicensed devices at EIRP up to 4 W, and this for any sensing/transmission antenna heights because of the reciprocity of the transmission path (i.e., the sensing radius is maintained larger than the interference radius). [5, 6, 7, 8, 9] Detailed propagation analyses have shown that, considering the reciprocity of the RF path between the interfering/sensing CPE and the 802.22.1 beacon, including an extra 5.7 dB margin for frequency selective fading due to the narrower bandwidth of the beacon compared to the 6 MHz bandwidth of the TVBD signal[1], the sensitivity required to detect this 802.22.1 beacon needs to be –116 dBm so that the sensing range of the 4 W EIRP TVBD is at least equal to its interference range [9].

A more detailed analysis of the sensing performance achievable with the 802.22.1 beacon led to the graph shown in Figure 1. Different levels of sensitivity are available depending on which part of the 802.22.1 beacon are detected and over which specific sensing durations. A key data point on the graph is the detection of the synchronization burst and index that can be done over a period of 5.1 ms. This detection allows the localization of the next burst that will carry more detailed information such as the geolocation of the beacon and the signature and authentication of the beacon. Acquiring this information will however take a larger quiet period and is left to the discretion of the TVBD operator. The beacon can also be detected quickly by identifying its spectrum spreading sequence. This can be done within 1ms while still providing the spread spectrum signature that will distinguish it from a spurious signal. Energy detection can be done on the beacon but a ‘de-spread’ energy detection will provide much better sensitivity as indicated in Figure 1. [9]

[pic]

Figure 1: Sensing performance of different parts of the 802.22.1 beacon

4- Other advantages of using the 802.22.1 wireless microphone beacon

Beyond the more reliable detection of wireless microphone operation in TV white space afforded by the use of the 802.22.1 beacon, this will minimize the occurrence of false positive due to the authentication capability of the beacon. Rather than relying on a highly variable level of wireless microphone signal due to the uncontrollable use of these devices in practical situation, an 802.22.1 beacon would normally be mounted in an advantageous location, above the wireless microphone receivers, to improve the interference protection afforded (e.g., beacon antenna on top of an ENG van, located higher in a sport stadium, etc. to maximize the extent of the ‘bubble’ of protection). The use of the 802.22.1 beacon with its specific digital signature will also avoid mistaking the presence of spurious signals and noise in the channel for wireless microphone operation. The use of an 802.22.1 beacon would also ease the requirement for the sensing time since the microphone operator or news crew would normally turn on the beacon a few minutes before being “on-the-air”.

Wireless microphone sensing could lead to excessively easy denial of service resulting from mocked wireless microphone signals or even the presence of very simple, very low power and easily accessible RF oscillators in the TV channel. Such denial of service for TVBDs would be rendered more difficult with the digital encoding and spectrum spreading schemes as well as authentication and certification included in the 802.22.1 beacon. The use of the 802.22.1 beacon will avoid any low power frequency modulated devices having precedence over other ‘unlicensed devices’.

Furthermore, since there is no modulation format enforced by the FCC R&O for wireless microphones, it will be very easy to create a TVBD with a "microphone signature" included in its spectral output (such as a narrowband modulated carrier transmitting useful data so that it would not be considered as wilful jamming, ‘spoofing’) to capture a band as a pseudo-incumbent, and gaining some form of "priority access" on the medium by making the other TVBDs devices believe that there is active wireless microphone operation in the area while none is present.

5- Insufficient protection resulting from wireless microphone sensing

The 4 W fixed TVBDs with their large radius of interference need to sense wireless microphones over a large area and this cannot be done reliably by simply sensing wireless microphones with their typical low power and high variability, especially in the case of wearable units. By extending the propagation studies that were undertaken to establish the modulation parameters for the 802.22.1 wireless microphone beacon, it is found that sensing a 10 mW wireless microphone at –114 dBm from a 4 W TVBD would miss the target of detecting a wireless microphone over a radius that is at least as large as the interfering radius by:

10*log(250) + 2 dBi –10*(log(10) – 6 + (-114-116) = 12 dB mainly due to the lower microphone transmission power compared to the nominal 250 mW beacon transmit power and 2 dBi whip antenna and the fact that the 6 dB margin needed to cover for the different extent of frequency selective fading between the 77 kHz beacon and the 6 MHz TVBD signal does not need to be taken into account since the actual wireless microphone would be sensed.

Given the fact that wireless microphone operation needs to be protected even in the case of the use of an antenna with –10 dBi gain and a 27 dB fading (body absorption and multipath), still giving a –95 dBm signal level that needs to be protected at 100 m as indicated in section 1 above, trying to protect wireless microphones by sensing them would miss the target by 12 dB to 49 dB in the case of 4 W EIRP fixed TVBDs. In the case of 100 mW personal/portable TVBDs, the target would still be missed by as much as 33 dB in the case of the worst microphone signal fading (- 95 dBm received signal). Sensing the 802.22.1 beacon would avoid this practical problem since the beacon was designed to make sure that the sensing range of the 4 W EIRP TVBD is at least equal to its interference range. The detection of the 802.22.1 beacon by the lower power 100 mW TVBDs will provide a 14 dB safety margin for the protection of wireless microphones because of the reduced interference range of these lower power devices if the same sensitivity of –116 dBm achievable through sensing times as depicted in Figure 1 above. Furthermore, the operator of the beacon can easily maximize the ‘bubble’ of protection by installing the beacon in a prominent place relative to the wireless microphone receivers.

If the number of local wireless microphone receivers increases, the redundancy resulting on the sensing of these microphones will tend to decrease the advantage of the 802.22.1 beacon (i.e., the microphones will not be faded by 27 dB at the same time as seen by the sensor.)

References:

[1] 22-04-0002-17-0000_WRAN_Reference_Model.xls, “WRAN Reference Model”, Tab: ‘CPE=>W-Micro’ Gerald Chouinard, CRC Canada , January 2009

[2] OET Report FCC/OET 08-TR-1005, “Evaluation of the Performance of Prototype TV-Band White Space Devices Phase II”, Technical Research Branch Laboratory Division, Office of Engineering and Technology, Federal Communications Commission, October 2008)

[3] P802.22.1/D4, “Part 22.1: Standard to Enhanced Harmful Interference Protection for Low-Power Licensed Devices Operating in the TV Broadcast Bands,” July 2008.

[4] P802.22/D1, Draft Standard for Wireless Regional Area Networks, “Part 22: Cognitive Wireless RAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Policies and procedures for operation in the TV Bands”, April 2008

[5] 22-08-0104-00-0001_TG1_PER_in_AWGN_and_WRAN-B.ppt, “IEEE 802.22.1 Packet Error Rates

for MSF1 and MSF2”, Stephen Kuffner, Motorola, March 2008

[6] 22-08-0042-01-0001_Simulation Results for TG1 according to Draft 2.01.ppt, “Simulation Results for TG1 according to Draft 2.0”, Yuchun Wu, Huawei HiSi, March 2008

[7] 22-08-0059-00-0001_TG1-link-margin-by-2-methods.doc, “Beacon Link Margin with Two Methods”, Stephen Kuffner, Motorola, February 2008

[8] 22-07-0148-00-0001_TG1_link_margin_calculator.xls, “802.22 TG1 Link Margin Calculator”, Stephen Kuffner, Motorola, March 2007

[9] 22-08-0040-01-0000-WRAN and TG1 Beacon link analysis.xls, “WRAN and TG1 Beacon link analysis”, Tab: ‘CPE to Beacon’, Gerald Chouinard, CRC Canada, March 2009

[10] 22-07-0491-06-0001-wran-annex-on-tg1-detection.doc, “Annex on TG1 detection for 802.22 draft”, Jinxia Cheng et al, March 2009

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[1] A frequency selective fade margin of 5.7 dB for the 802.22.1 beacon locatred at 3 m above ground offers a 99.9% link availability according to the ITU-R Rec. P.1546 in rural open area condition and 98% according to the Okumura/Hata model for small-medium city environment. [9]

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Abstract

This document analyzes the improved sensing performance achievable with the use of the 802.22.1 wireless microphone beacon as compared to sensing wireless microphones directly. Considerations related to potential denyal of service for unlicensed devices in the TV white spaces and the potential improvement coming from the 802.22.1 beacon signature and authentication are also discussed.

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